Incrementing a value with mongoose?

I have a mongoose model in my node.js application, representing invoices. I have figured most of it out already, but I really need to ensure that my invoices are numerated/incremented to be able to provide a proper reference to my customer.

Using an SQL database, I would have created an AUTO-INCREMENT column holding this value, but this obviosly isn't built into MongoDB. So how would I accomplish this with mongoose?

Here's how my model looks right now:

var InvoiceSchema = new Schema({
    reference: {type: Number, default: 0}, // The property I want to auto-incr.

    dates: {
        created:  {type: Date, default: Date.now},
        expire: {type: Date, default: expiryDate()}
    },

    amount: {type: Number, default: 0}
    // And so on
});